Page 211 - Fluid Catalytic Cracking Handbook

Unit Monitoring and Control 179

To maximize the unit's profit, one must operate the unit simul-
taneously against as many constraints as possible. Examples of these
constraints are limits on the air blower, the wet gas compressor,
reactor/regenerator temperatures, slide valve differentials, etc. The
conventional regulatory controllers work only one loop at a time and
they do not talk to one another. A skilled operator can "push" the unit
against more than one constraint at a time, but the constraints change
often. To operate closer to multiple constraints, a number of refiners
have installed an advanced process control (APC) package either
within their DCS or in a host computer.
The primary advantages of an APC are:

* It provides more precise control of the operating variables against
the unit's constraints and, therefore, obtains incremental through-
put or cracking severity.
* It is able to respond quickly to ambient disturbances, such as cold
fronts or rainstorms. It can run a day/night operation, taking
advantage of the cooler temperatures at night.
* It pushes against two or more constraints rather than one single
constraint. It can maximize the air blower and wet gas compressor
capacities.
As mentioned above, there are two options for installing an APC.
One option is to install an APC within the DCS framework, and the
other is to install a multivariable modeling/control package in a host
computer. Each has advantages and disadvantages, as indicated below,

Advantages of Multivariable Modeling and Control

The multivariable modeling/control package is able to hold more
tightly against constraints and recover more quickly from disturbances.
This results in an incremental capacity used to justify multivariable
control. An extensive test run is necessary to measure the response
of unit variables.
In APC on DCS framework, the control structure must be designed,
configured, and programmed for each specific unit. Modifying the
logic can be an agonizing process. Wiring may be necessary. It is
difficult to both document the programming and to test.